This invention relates to an aerial prospecting system, and particularly to a radioactive material prospecting system.
Prospecting for radioactive materials usually involves scanning a particular land area from the air with a gamma ray spectrometer for the detection of gamma rays from radioactive material. Systems in common use utilize NaI crystals which release photons of energy upon gamma rays colliding with their atoms. These photons are detected in photomultiplier tubes, and the resulting output signals can be used to indicate the intensity of the gamma ray emissions.
Such systems utilize altimeters to record the height of the airplane from the ground, real time clocks which, used in conjunction with the known velocity of the airplane, can provide an indication of the location of any anomaly, etc.
Such systems typically group the energy spectrum into energy windows (which, in gamma spectrometry is a group of energy channels, typically over 127) and record on paper by pen chart and/or on magnetic tape the intensity of the gamma rays within each of the energy spectrum windows.
Such systems have encountered particular problems. For example gamma rays have been found to be emitted from the earth itself, which are not particularly related to a concentrated source of radioactive material such as uranium. This background radiation which is caused by gamma rays scattered in the ground, air or within the NaI crystal itself is called "Compton background" and is generally taken into account as being a constant proportion of counts in higher energy windows. It is known, however, that the Compton background radiation varies with variations in the amount of matter between the source of gamma rays and the detector.
As the spectrometer scans a given area, the geophysicist looks for anomalies in the energy windows of the received spectrum. These anomalies are generally manifested by an increase in amplitude above the Compton background at particular energy windows within the spectrum. The time of persistence, and the peak amplitude or area which is detected above the Compton background allows the geophysicist to estimate the size and concentration of the radioactive anomaly.
Particular spectrogram windows have been identified in the past as containing evidence of radioactive material. For instance, uranium, thorium, potassium.sup.', are all natural radioactive elements which emit gamma rays within different window areas of a spectrogram.
As the gamma rays collide with the NaI crystal, a certain proportion may be scattered and do not spend all their energy within the NaI crystal. Also some gamma rays have previously interacted with atoms in the rock or soil, thus losing part of their energy and do not enter the crystal with their original energy. The gamma rays which did not lose all their energy within the crystal create Compton background and the number of counts of Compton background increases with decreasing energy level and with the amount of mass between the radioactive source and the detector.
Gamma rays which lose all their energy within the crystal manifest themselves in the spectrum as "photopeaks" with the photopeak amplitude (or areas below a graphical plot of the photopeak) being proportional to the intensity of the particular gamma rays at their location in the spectrum corresponding to their energy.
Compton background can therefore be considered as "noise", while photopeaks represent "signal". One of the most important tasks of airborne gamma spectrometry is to separate photopeak signals from Compton noise.
The various channels of energy level which are recorded must therefore be corrected for the everpresent Compton background. This process normally utilizes a correction technique called Compton stripping.
It has been found that while previous correction techniques takes Compton background as a constant, that Compton background is actually variable, and, according to prior art techniques, may mask photopeaks of main interest. Accordingly prior art Compton background "stripping " techniques have caused the geophysicist to make, in many instances, erroneous conclusions as to the presence of radioactive anomalies. It has been found that if Compton background correction is treated as a constant as in the prior art, it can create false anomalies or obliterate real ones.
The present invention provides a new apparatus and an improved process for obtaining indications of photopeaks which were previously removed during above mentioned Compton stripping technique.